The present invention relates more particularly, although by no means exclusively, to a method of starting a molten bath-based smelting process for producing molten metal from a metalliferous feed material in a smelting vessel that has a strong bath/slag fountain generated by gas evolution in the molten bath, with the gas evolution being at least partly the result of devolatilisation of carbonaceous material into the molten bath.
In particular, although by no means exclusively, the present invention relates to a method of starting a process for smelting an iron-containing material, such as an iron ore, and producing molten iron.
The present invention relates particularly, although by no means exclusively, to a method of starting a smelting process in a smelting vessel that includes a main chamber for smelting metalliferous material.
A known molten bath-based smelting process is generally referred to as the HIsmelt process, is described in a considerable number of patents and patent applications in the name of the applicant.
Another molten bath-based smelting process is referred to hereinafter as the “HIsarna” process. The HIsarna process and apparatus are described in International application PCT/AU99/00884 (WO 00/022176) in the name of the applicant.
The HIsmelt process and the HIsarna processes are associated particularly with producing molten iron from iron ore or another iron-containing material.
In the context of producing molten iron, the HIsmelt process includes the steps of:                (a) forming a bath of molten iron and slag in a main chamber of a smelting vessel;        (b) injecting into the bath: (i) iron ore, typically in the form of fines; and (ii) a solid carbonaceous material, typically coal, which acts as a reductant of the iron ore feed material and a source of energy; and        (c) smelting iron ore to iron in the bath.        
The term “smelting” is herein understood to mean thermal processing wherein chemical reactions that reduce metal oxides take place to produce molten metal.
In the HIsmelt process solid feed materials in the form of metalliferous material and solid carbonaceous material are injected with a carrier gas into the molten bath through a number of lances which are inclined to the vertical so as to extend downwardly and inwardly through the side wall of the main chamber of the smelting vessel and into a lower region of the vessel so as to deliver at least part of the solid feed materials into the metal layer in the bottom of the main chamber. The solid feed materials and the carrier gas penetrate the molten bath and cause molten metal and/or slag to be projected into a space above the surface of the bath and form a transition zone. A blast of oxygen-containing gas, typically oxygen-enriched air or pure oxygen, is injected into an upper region of the main chamber of the vessel through a downwardly extending lance to cause post-combustion of reaction gases released from the molten bath in the upper region of the vessel. In the transition zone there is a favourable mass of ascending and thereafter descending droplets or splashes or streams of molten metal and/or slag which provide an effective medium to transfer to the bath the thermal energy generated by post-combusting reaction gases above the bath.
Typically, in the case of producing molten iron, when oxygen-enriched air is used, it is fed at a temperature of the order of 1200° C. and is generated in hot blast stoves. If technically pure cold oxygen is used, it is typically fed at or close to ambient temperature.
Off-gases resulting from the post-combustion of reaction gases in the smelting vessel are taken away from the upper region of the smelting vessel through an off-gas duct.
The smelting vessel includes refractory-lined sections in the lower hearth and water cooled panels in the side walls and the roof of the main chamber of the vessel, and water is circulated continuously through the panels in a continuous circuit.
The HIsmelt process enables large quantities of molten iron, typically at least 0.5 Mt/a, to be produced by smelting in a single compact vessel.
The HIsarna process is carried out in a smelting apparatus that includes (a) a smelting vessel that includes a main smelting chamber and lances for injecting solid feed materials and oxygen-containing gas into the main chamber and is adapted to contain a bath of molten metal and slag and (b) a smelt cyclone for pre-treating a metalliferous feed material that is positioned above and communicates directly with the smelting vessel.
The term “smelt cyclone” is understood herein to mean a vessel that typically defines a vertical cylindrical chamber and is constructed so that feed materials supplied to the chamber move in a path around a vertical central axis of the chamber and can withstand high operating temperatures sufficient to at least partially melt metalliferous feed materials.
In one form of the HIsarna process, carbonaceous feed material (typically coal) and optionally flux (typically calcined limestone) are injected into a molten bath in the main chamber of the smelting vessel. The carbonaceous material is provided as a source of a reductant and a source of energy. Metalliferous feed material, such as iron ore, optionally blended with flux, is injected into and heated and partially melted and partially reduced in the smelt cyclone. This molten, partly reduced metalliferous material flows downwardly from the smelt cyclone into the molten bath in the smelting vessel and is smelted to molten metal in the bath. Hot reaction gases (typically CO, CO2, H2, and H2O) produced in the molten bath is partially combusted by oxygen-containing gas (typically technical-grade oxygen) in an upper part of the main chamber. Heat generated by the post-combustion is transferred to molten droplets in the upper section that fall back into the molten bath to maintain the temperature of the bath. The hot, partially-combusted reaction gases flow upwardly from the main chamber and enter the bottom of the smelt cyclone. Oxygen-containing gas (typically technical-grade oxygen) is injected into the smelt cyclone via tuyeres that are arranged in such a way as to generate a cyclonic swirl pattern in a horizontal plane, i.e. about a vertical central axis of the chamber of the smelt cyclone. This injection of oxygen-containing gas leads to further combustion of smelting vessel gases, resulting in very hot (cyclonic) flames. Finely divided incoming metalliferous feed material is injected pneumatically into these flames via tuyeres in the smelt cyclone, resulting in rapid heating and partial melting accompanied by partial reduction (roughly 10-20% reduction). The reduction is due to both thermal decomposition of hematite and the reducing action of CO/H2 in the reaction gases from the main chamber. The hot, partially melted metalliferous feed material is thrown outwards onto the walls of the smelt cyclone by cyclonic swirl action and, as described above, flows downwardly into the smelting vessel below for smelting in the main chamber of that vessel.
The net effect of the above-described form of the HIsarna process is a two-step countercurrent process. Metalliferous feed material is heated and partially reduced by outgoing reaction gases form the smelting vessel (with oxygen-containing gas addition) and flows downwardly into the smelting vessel and is smelted to molten iron in the smelting vessel. In a general sense, this countercurrent arrangement increases productivity and energy efficiency.
The HIsmelt and the HIsarna processes include solids injection into molten baths in smelting vessels via water-cooled solids injection lances.
In addition, a key feature of both processes is that the processes operate in smelting vessels that include a main chamber for smelting metalliferous material and a forehearth connected to the main chamber via a forehearth connection that allows continuous metal product outflow from the vessels. A forehearth operates as a molten metal-filled siphon seal, naturally “spilling” excess molten metal from the smelting vessel as it is produced. This allows the molten metal level in the main chamber of the smelting vessel to be known and controlled to within a small tolerance—this is essential for plant safety. Molten metal level must (at all times) be kept at a safe distance below water-cooled elements such as solids injection lances extending into the main chamber, otherwise steam explosions become possible. It is for this reason that the forehearth is considered an inherent part of a smelting vessel for the HIsmelt and the HIsarna processes.
The term “forehearth” is understood herein to mean a chamber of a smelting vessel that is open to the atmosphere and is connected to a main smelting chamber of the smelting vessel via a passageway (referred to herein as a “forehearth connection”) and, under standard operating conditions, contains molten metal in the chamber, with the forehearth connection being completely filled with molten metal.
The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.
During the course of pilot plant trials of the HIsarna process it became necessary for the applicant to make an unscheduled end-tap of the smelting vessel used in the trials. Molten metal was removed successfully from the main chamber of the smelting vessel in the end-tap, but substantially all of the molten slag remained behind and solidified in the smelting vessel. This resulted in the main chamber, the forehearth connection and the forehearth of the smelting vessel being filled with cold (frozen) slag to a level above the level of the forehearth connection between the forehearth and the main chamber of the smelting vessel.
Standard process start-up for the HIsmelt process and the proposed process start-up for the HIsarna process involve establishing a molten metal bath in the smelting vessel by pouring a charge of fresh molten metal into the vessel via the forehearth and the forehearth connection. Before the pilot plant could be started again, it was therefore necessary to re-establish a clear connection between the forehearth and the smelting vessel. A standard option of allowing the entire system to cool down and then mechanically digging out frozen slag was considered by the applicant to be too time-consuming and therefore not a preferred option.